Chemical Reactors

Chemical Engineering

Reactor Classification and type selection

Homogenous Reactors:Only one phase is involved: gas or liquid When more phases are involved the mixing is very importantHeterogeneous Reactors:Two or more phases are involved.Normally solids if present then are in catalyst form.A heterogeneous reactor may involve a heterogeneous reaction or

a homogeneous reaction.

Heterogeneous reactors show greater variety of configuration and contacting patterns than homogenous reactors.

Types of Reactors

Batch Reactor (BR, STR)The reactants are initially charged into the vessel and are well mixed and left to react for a certain period of time. The resultant mixture is then discharged. This is an unsteady operation where the composition changes with time but is uniform throughout the reactor at a specific time.

Continuous Reactors

Continuous stirred tank reactor (CSTR, MFR, BMFR)– An agitator is introduced to disperse the reactants thoroughly

into the reaction mixture immediately they enter the reactor.– Product is continuously drawn out and that’s why known for

perfect mixing.– Compositions at outlet and inside reactor are same.– Best suitable for liquid phase reactions

Plug Flow Reactor: (PFR)– These are tubular reactors generally but not necessarily.– Often called piston flow, slug flow, ideal tubular flow or

unmixed flow reactors.– The residence time for all the elements is same: Must be– Sometimes used for liquid phase reactions but best suited for

gas phase reactions.

Semi-Batch Reactors

– Aim of their invention is to get benefit of any thing by changing the contacting pattern.

– One of the reactants may not be charged at once but slowly.When required:

To react a gas with a liquidTo control a highly exothermic reactionTo improve the product yield in suitable circumstances

Heat of Reaction and Reactor Type

• When heat of Reaction is too small then can be neglected• But if it is high then this is major influencing factor• The temperature of the system can rise or fall depending

upon the reaction type: Exothermic or Endothermic

• A relation must be there among enthalpy, heat transferred, and temperature change of the system (Energy balance)

Before designing, check for:1- What is the heat of reaction?2- Acceptable range of temperature?

Different reactor configurations based on Temperature(Modes of Operation)

Adiabatic Reactors

• Very easy to design• Temperature drop or rise remains within acceptable range• The properties of the product are not affected by such rise or

fall in temperature

Reactors with Heat TransferThis arrangement is used when isothermal operation is desiredThe temperature can be controlled through internal coils,

external jackets or external heat exchangerThe factors which influence the H.T. are:

1-H.T. coefficient2- Jacket Pressure3- Jacket Pressure drop4- Reactor pressure5- Cleanliness6- Cost

Reactor with Internal Coils

Jacketed Reactor and its types

Reactor with heat exchanger

Auto thermal Reactors

• These are the self supporting systems in which heat of one stream is used to heat feed stream in order to raise the reaction rate and save the time and the cost.

• These show integrated reactor system with feed back systems• An external source of heat is required to start the reaction

once and then reaction proceeds on itself.• This is valid for Highly Exothermic Reaction Systems

Choice of Process Conditions

Two main principles are involved:1- Chemical Equilibrium2- Chemical KineticsIf equilibrium contact is very large, then reaction is said to be irreversible. But there lies a max. extent of that reaction upto which it can proceed (Chem. Equil.).How to proceed: Find the applicable temperature range of reaction and then investigate Kc under that temperature range.

From recommended literature it is found that: